The invention pertains to a method for monitoring an entrance to a hazardous area.
Light barriers, light curtains, light screens, etc., have been used for monitoring hazardous areas, e.g., the working area of a machine, for quite some time. The light barriers are usually arranged at entrances to the hazardous area and serve primarily for protecting persons and generally for monitoring if an object that is considered xe2x80x9cadmissible,xe2x80x9d e.g., a workpiece to be processed or a workpiece carrier, travels into the hazardous area and is properly aligned or if the object consists of an xe2x80x9cinadmissiblexe2x80x9d object. Depending on the object shape and the object size, individual light barriers are interrupted or blanked or partially blanked when the object passes the entrance. In this case, a series of light barriers would, in principle, make it possible to realize an xe2x80x9cobject recognitionxe2x80x9d by comparing a sensed xe2x80x9csignal patternxe2x80x9d with a predetermined signal pattern that is considered admissible. If the size and the position of the object, e.g., a workpiece carrier, are known, the same light beams are always blanked or partially blanked if the object is properly aligned. However, this is not always the case due to misaligned workpieces, vibrating machine components and vibrating light barrier components, scattered light, etc. This means that, in particular, xe2x80x9cmarginal beams,xe2x80x9d i.e., light beams that extend in marginal regions of an object to be scanned, are sometimes blanked more intensely than at other times.
The invention is based on the objective of disclosing a monitoring method that eliminates the aforementioned problems.
The invention is based on a comparison between a signal pattern generated by several light barriers and at least one predetermined signal pattern. In this case, the sensed signal pattern is also considered to be admissible if the signal states of light barriers, the light beams of which extend in the marginal region of an object to be scanned, deviate from the predetermined signal pattern. In simpler terms, this means that, if a marginal light beam is blanked more intensely with reference to a predetermined signal pattern, a marginal light beam that extends on the opposite side of the object needs to be blanked less intensely or be free and vice versa.
The light barriers are arranged at the entrance to a hazardous area and are interrupted, diminished by object edges, or not influenced when the object passes. This results in a xe2x80x9csignal patternxe2x80x9d that depends on the size, shape and alignment of the object.
Depending on the respective application, the sensed signal pattern may deviate from the predetermined signal pattern, e.g., due to a xe2x80x9cchangedxe2x80x9d object position, shaking of a workpiece support or movements of a workpiece to be scanned in the protected area, vibrations of the object and of components of the light barriers, etc. Consequently, the xe2x80x9cpartial blankingxe2x80x9d of individual light beams may change and xe2x80x9cmarginal light beams,xe2x80x9d i.e., light beams that extend on object edges, may become entirely free or interrupted.
Since objects, the signal pattern of which does not exactly correspond to predetermined signal patterns, are also considered to be admissible in accordance with the invention, errors in the object recognition are reduced such that a higher xe2x80x9crobustnessxe2x80x9d of the monitoring device is achieved and a superior utilization of the capacity of the machine to be monitored and shorter machine standstill times can be realized.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.